The present invention relates to electric storage cells of alkaline type and is concerned with such cells, or batteries made up from such cells, which are of sealed type. During operation, and more particularly during the latter stages of charging, gases, normally hydrogen and/or oxygen, are liberated in such cells, and the term "sealed" designates a cell which has no vent for the release of such gas at atmospheric pressure. This term is not used to exclude the provision of a safety valve or of a vent which opens to vent the interior of the cell at some predetermined pressure above atmospheric pressure.
The evolution of gas from the positive or negative plates occurs primarily on charging when those plates are fully charged and oxygen is liberated at the positive plates whilst hydrogen is liberated at the negative plates. It is found that evolved oxygen can pass to the negative plates, which includes cadmium in a nickel/cadmium cell, and tend to recombine there and thus not contribute to an increase in pressure of the cell. In so doing it partially discharges the negative plates and suppresses hydrogen evolution. If hydrogen is evolved it does not tend to recombine similarly and for this reason it is usual in such cells to provide an excess of negative active material so that the positive plates become charged before the negative plates and thus in practice little or no hydrogen is evolved.
It is desirable that as high a proportion as possible of the oxygen evolved in the cell be induced to recombine within the cell since a loss of oxygen and accompanying hydrogen represents a loss of electrolyte which must be compensated for by topping up the electrolyte. However, the conditions which promote effective recombination of the oxygen militate against the effective electrochemical operation of the cell at high rates of discharge.
In conventional pocket plate alkaline cells which are fully flooded with electrolyte, adjacent plates are separated by separators whose sole function is to space the plates apart thus preventing short circuits and do not have the subsidiary functions associated with the separators in lead acid cells, namely to prevent the shedding of active material and to inhibit "treeing through", that is to say the growth of lead dendrites from the plates which can ultimately lead to short-circuits. Thus in such alkaline cells the separators conventionally have as large as possible an open area as in consistent with preventing contact of the plates so as to minimise the electrical resistance between adjacent plates. This resistance is largely responsible for the internal resistance of the cell and it must be kept as low as possible if the battery is to be efficient and is to have an acceptable high rate discharge performance.
Recombination of oxygen on a negative plate occurs by diffusion of oxygen through the layer of electrolyte present on the surface of the plate. The rate at which recombination can occur is therefore dependent upon the rate at which oxygen can diffuse through this layer and this in turn is dependent upon the thickness of this layer. Thus, in a fully flooded cell with a layer of electrolyte between the plates of the order of 1 mm-2 mm thick, the rate of recombination is very low. In a conventional sealed cell the recombination rate is increased substantially be using an electrolyte absorbent separator which is just moist with electrolyte. By this means the thickness of the layer of electrolyte covering the negative plate is kept very small and large rates of recombination can be sustained. However, this absorbent type separator has a much lower open area than that of a conventional flooded cell and therefore has a much higher resistance. In consequence a pocket plate cell with an absorbent separator has a high rate discharge voltage some 200-300 mV lower than a cell with a conventional separator, even when the former separator is in the fully saturated condition.
It is therefore an object of the invention to provide an electric storage cell of alkaline type in which substantially all the gas evolved within the cell is induced to recombine, at least at relatively low charging rates of, for instance between C/60 and C/1000, but whose performance is not substantially degraded by comparison with similar cells of the same volume.